The long term objective of this proposal is to determine the methods by which Theiler's murine encephalomyelitis virus (TMEV) is able to persist in mice for long periods of time, the result of which is a chronic demyelinating disease similar to human multiple sclerosis. A quantitative analysis of TMEV persistent infection is proposed. A highly sensitive real time reverse transcriptase-PCR (RT-PCR) method will be developed and utilized for detection of low level virus in extra-neural tissues and in mutant virus infection. Using purified spinal cord cells, RT-PCR will be used to determine the number and types of cells infected during the persistent stage of disease. Moreover, TMEV infection of specific macrophage subpopulations will be determined. Finally, specific persistence determinants in the viral coat will be analyzed by mutagenesis. The proposed experiments will provide insight into the mechanism by which TMEV persists in mice despite an immune response directed against it. Such a study has relevance not only to multiple sclerosis, but also to other persistent viruses, such as human immunodeficiency virus and hepatitis C virus.